Organic light-emitting devices (OLEDs) or organic electroluminescent (EL) devices have been known for several decades, however, their performance limitations have represented a barrier for many applications. In the simplest form, an OLED is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination and emission of light. Representative of earlier OLEDs can be found in the RCA Review, 30, 322 (1969), entitled “Double Injection Electroluminescence in Anthracene” and U.S. Pat. Nos. 3,172,862; 3,173,050; and 3,710,167. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 μm). Consequently, operating voltages were very high, often >100V.
More recent OLEDs include an organic EL medium containing extremely thin layers (e.g. <1.0 μm) between the anode and the cathode. Herein, the term “organic EL medium” encompasses the layers between the anode and cathode. Reducing the thickness has lowered the resistance of the organic layers and enabled devices that operate at much lower voltage. In a basic two-layer OLED structure, described first in U.S. Pat. No. 4,356,429 by Tang, one organic layer of the EL medium adjacent to the anode is specifically selected to transport holes, and therefore is referred to as the hole-transporting layer (HTL), and the other organic layer is specifically selected to transport electrons and is referred to as the electron-transporting layer (ETL). Recombination of the injected holes and electrons within the organic EL medium, results in efficient electroluminescence.
Based on the two-layer OLED structure, numerous OLEDs with alternative layer structures have been disclosed. For example, there are three-layer OLEDs that contain an organic light-emitting layer (LEL) between the HTL and the ETL, such as that disclosed by Adachi et al., “Electroluminescence in Organic Films with Three-Layer Structure”, Japanese Journal of Applied Physics, 27, L269 (1988), and by Tang et al., “Electroluminescence of Doped Organic Thin Films”, Journal of Applied Physics, 65, 3610 (1989). The LEL commonly include a host material doped with a guest material, otherwise known as a dopant. Further, there are other multilayer OLEDs that contain additional functional layers such as a hole-injecting layer (HIL), an electron-injecting layer (EIL), an electron-blocking layer (EBL), or a hole-blocking layer (HBL) in the devices. These new structures have resulted in improved device performance.
Many light-emitting materials emit light from their excited singlet state by fluorescence. The excited singlet state can be created when excitons formed in an OLED transfer their energy to the singlet excited state of the dopant. However, only 25% of the excitons created in an OLED are singlet excitons. The remaining excitons are triplets, which cannot readily transfer their energy to the dopant to produce the singlet excited state of a dopant. This results in a large loss in efficiency since 75% of the excitons are not used in the light emission process.
Triplet excitons can transfer their energy to a dopant if the dopant has a triplet excited state that is low enough in energy. If the triplet state of the dopant is emissive it can produce light by phosphorescence. In many cases, singlet excitons call also transfer their energy to the lowest singlet excited state of the same dopant. The singlet excited state can often relax, by an intersystem crossing process, to the emissive triplet excited state. By the proper choice of host and dopant, energy can be collected from both the singlet and triplet excitons created in an OLED. The term electrophosphorescence is sometimes used to denote EL wherein the mechanism of luminescence is phosphorescence; the term phosphorescent OLED is used to denote the OLED that can produce electrophosphorescence.
From device architecture perspective, in order to achieve improved quantum efficiency in a phosphorescent OLED, blocking layer, either hole-blocking layer or electron-blocking layer, has been used in confining carriers and excitons within a light-emitting layer. For example, Baldo et al in U.S. Pat. No. 6,097,147 teach to form a hole-blocking layer, such as bathocuprione (BCP), between a light-emitting layer and an electron-transporting layer in a phosphorescent OLED. Thompson et al. in U.S. Pat. No. 6,951,694 disclose phosphorescent OLEDs comprising an electron-blocking layer and a light-emitting layer with a neat host and a phosphorescent dopant. By inserting an electron-blocking layer between the hole-transporting layer and the light-emitting layer electron leakage can be eliminated and, hence, luminous efficiency is increased. Fac-tris (1-phenylpyrazolato, N,C2′)Iridium(III)(Irppz) and Iridium(III)bis(1-phenylpyrazolato, N,C2′)(2,2,6,6-tetramethyl-3,5-heptanedionato-O,O) (ppz2Ir(dpm)) have been disclosed as suitable electron-blocking materials.
Recently, Kondakova et al in US 2006/134,460 A1 disclose phosphorescent OLEDs comprising an exciton-blocking layer and a co-host light-emitting layer having at least one hole-transporting host and at least one electron-transporting host, together with at least one phosphorescent dopant. The exciton-blocking layer in the devices includes a hole-transporting material with triplet energy greater or equal to 2.5 eV adjacent the light-emitting layer on the anode side.
Notwithstanding all these developments, there remains a need to further improve quantum efficiency and operational lifetime of the phosphorescent OLEDs.